In certain thermal joining processes, such as brazing or soldering, operation outside of inert environments requires a flux to prevent oxides from forming while the metal is heated, in addition to cleaning any contamination left on the joined surfaces. The flux may be applied using paste, liquid, powder, or joining material rods with a flux coating. During a thermal cycle, the flux melts first into the joint and is displaced by a filler material, e.g., braze alloy, and occupies the joint.
Fluxes can vary in performance, and are generally selected in view of the particular base metal(s) being joined. For example, phosphorus-containing brazing alloys can be self-fluxing when joining copper to copper, e.g., BCup-5, but braze alloys containing phosphorus produce brittle phosphides if used with iron or nickel base metals. However, in other situations a non-phosphorus alloy may be required where high corrosion life is a requirement. Typically, the flux should be chemically compatible with both the joining alloy and the base metal being used to prevent preferential corrosive attack.
Copper joints requiring non-phosphorus joining alloys contain silver, i.e., Ag, for corrosion resistance. Typically, non-phosphorus joining alloys require a controlled environment to prevent oxide contamination and joint porosity. For example, contemporary brazing systems utilize brazing chambers on the factory floor for part sub-assembly. Parts cannot be assembled at the point of use, e.g., in the field or during assembly line installation, or brazed in position due to the size and complexity inherent in chamber brazing restrictions. Consequently, these situations increase labor costs and decrease production throughput.